Electric Motors

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Electric Motors Types of Electric Motors
North Seattle Community College HVAC
Program Instructor Mark T. Weber, M.Ed., CMHE
Elec motors - 2
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Objectives

• After studying this unit, you should be able to
• Describe the different types of open single-phase
  motors used to drive fans, compressors, and pumps
• Describe the applications of the various types of
  motors
• State which motors have high starting torque

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Objectives (contd.)

• List the components that cause a motor to have a
  higher starting torque
• Describe a multispeed permanent split-capacitor
  motor and indicate how the different speeds are
  obtained
• Explain the operation of a three-phase motor
• Describe a motor used for a hermetic compressor

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Objectives (contd.)

• Explain the motor terminal connections in various
  compressors
• Describe the different types of compressors that
  use hermetic motors
• Describe the use of variable-speed motors

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Uses of Electric Motors

• Used to turn fans, pumps, compressors
• Facilitate the circulation of air, water,
  refrigerant, and other fluids
• Motors are designed for particular applications
• The correct motor must always be used
• Most motors operate on similar principles

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Fans are used to move air
Pumps are used to move liquids
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Parts of an Electric Motor

• Parts
• Stator with motor windings stationary portion of
  the motor
• Rotor rotating portion of the motor
• Bearings allow free rotation of the shaft
• End bells supports bearings and/or shaft
• Housing holds all motor components together and
  facilitates motor mounting

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Parts of an Electric Motor (contd.)
Figure 173 Individual electric motor parts
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Electric Motors and Magnetism

• Electricity and magnetism are used to create
  rotation
• Stator has insulated windings called run windings
• Rotor may be constructed of bars
• Squirrel cage rotor positioned between the run
  windings
• Rotor turns within the magnetic field

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Figure 174 Poles (north and south) on a rotating
magnet will line up with the opposite poles on a
stationary magnet
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Determining Motor Speed

• As the number of poles increases, the motor speed
  decreases
• Motor Speed (rpm) Frequency x 120 of poles
• In the United States, the frequency is 60 Hz
• For example, a two-pole motor will turn at a
  speed of 60 x 120 2 7200 2 3600 rpm

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Determining Motor Speed (contd.)

• The motor will turn at a speed that is lower than
  the calculated value
• Slip difference between calculated and actual
  motor speed

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Start Windings

• Enables the motor to start and in the right
  direction
• Start winding has higher resistance than the run
  winding
• Wound with more turns
• Wound with smaller diameter wire
• Removed from the active circuit once the motor
  starts

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Starting and Running Characteristics

• List of characteristics
• Refrigeration compressors have high starting
  torque
• Starting torque twisting force that starts the
  motor
• Locked Rotor Amperage (LRA)
• Full Load Amperage (FLA)
• Rated Load Amperage (RLA)
• Motor may start with unequal pressures across it

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Electrical Power Supplies

• Residences are furnished with single-phase power
• Houses can be supplied power from the transformer
• Power feeds into circuit breaker panel or fuse box

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Electrical Power Supplies (contd.)

• Circuit breakers protect each individual circuit
• Power is distributed throughout the house
• Typical residential panels provide 115 and 230
  volts
• Commercial and industrial facilities require
  three-phase power

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Figure 1711 A wiring diagram of a main circuit
breaker panel for a typical residence
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Single-Phase Open Motors

• Residential motors operate at 115, 208, or 230V
• Commercial motors operate at voltages up to 460V
• Some motors are designed to operate at one of two
  different voltage (dual voltage motors)
• Dual voltage motors are wired differently for
  each voltage
• Some motors have reversible rotations

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Figure 1714 The wiring diagram of a dual-voltage
motor. This motor is designed to operate at
either 115 V or 230 V, depending on how the motor
is wired in the field(A) A 230-V wiring diagram
(B) A 115-V wiring diagram
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Split-Phase Motors

• Two separate motor windings
• Good running efficiency
• Medium amount of starting torque
• Speed typically ranges from 1800 3600 rpm
• Motor speed is determined by the number of poles
• Slip is the difference between the calculated and
  actual motor speeds

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Split-Phase Motors (contd.)
Figure 1716 Diagram of the start and run windings
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The Centrifugal Switch

• Commonly used on open motors to de-energize the
  start winding
• Opens its contacts when the motor reaches about
  75 of its rated speed
• When the contacts open and close, a spark is
  created (arcing)
• Not used in a refrigerant atmosphere

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The Electronic Relay

• Used to open the start windings after the motor
  has started
• Solid-state device designed to open the start
  winding circuit when the design speed has been
  obtained

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Capacitor-Start Motors

• Split phase motor with start and run windings
• Start capacitor assists the motor starting by
  increasing the starting torque
• Start capacitor is wired in series with the
  motors start winding
• Start capacitor is removed from the circuit when
  the start winding is removed
• Start capacitor increases the phase angle

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Capacitor-Start Motors (contd.)
Figure 1722 Wiring diagram of a capacitor-start
motor
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Capacitor-Start, Capacitor-Run Motors

• Most efficient single-phase motor
• Often used with belt-driven fans and blowers
• Run capacitor improves running efficiency
• Run capacitor is in the circuit whenever the
  motor is energized
• Start and run capacitors are wired in parallel
• Motor amperage will rise if run capacitor goes
  bad

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Capacitor-Start, Capacitor-Run Motors (contd.)
Figure 1723 Wiring diagram of a capacitor-start,
capacitor-run motor. The start capacitor is in
the circuit only during motor start-up, whereas
the run capacitor is in the circuit whenever the
motor is energized
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Permanent Split Capacitor (PSC) Motors

• Simplest split-phase motor
• Only a run capacitor is used
• Low starting torque and good running efficiency
• Can be single or multispeed motors, and
  multispeeds have leads for each speed
• As resistance decreases, motor speed increases
• As resistance increases, motor speed decreases

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Permanent Split Capacitor (PSC) Motors (contd.)
Figure 1727 This diagram shows how the windings
of a three speed PSC motor are configured. As the
winding resistance increases, the motor speed
decreases
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Shaded-Pole Motors

• Very low starting torque
• Not as efficient as the PSC motor
• A portion of the run winding is shaded to provide
  the imbalance in magnetic field that allows the
  motor to start
• Heavy copper wire or bands are used to shade the
  run winding
• Manufactured in the fractional horsepower range

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Figure 1729 Wiring diagram of a shaded-pole motor
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Three-Phase Motors

• Normally used on commercial applications
• Must have a three-phase power supply
• Has no start winding or capacitors
• Very high starting torque
• Rotation of motor can be changed by switching any
  two power legs

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Three-Phase Motor (contd.)
Figure 1732 (A) Diagram of a three-phase power
supply (B) Diagram of a typical, single-speed,
three-phase motor
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Single-Phase Hermetic Motors

• Hermetically sealed from outside air
• Similar to single-phase motors
• Use relays to remove start winding
• They do not use centrifugal switches
• Use run capacitors for increased efficiency
• Designed to operate in a refrigerant atmosphere
• Motor terminals identified as common, start run

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The Potential Relay

• Used on motors requiring high starting torque
• Coil with very high resistance
• Normally closed contacts
• Relay operates on the induced voltage across the
  start winding
• The contacts open when the induced voltage rises
  when the induced voltage drops, the relay
  contacts close

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(A)
(C)
(B)
Figure 17-38 (A) A potential relay with its
packaging box (B) Internals of a potential relay
showing the coil and contacts (C) Diagram
illustrating the higher induced voltage that is
measured across the start winding in a typical
motor
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The Current Relay

• Used on fractional horsepower motors
• Used with fixed-orifice metering devices
• Low resistance coil in series with the run
  winding
• Normally open contacts in series with start
  winding
• Upon startup, only the run winding is energized

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The Current Relay (contd.)

• The motor draws locked rotor amperage
• The increased amperage closes the relay contacts
• The start winding is energized and the motor
  starts
• The amperage drops and the relay contacts open

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Figure 1739 Wiring diagram of a current magnetic
relay. The L indicates line voltage, the M
refers the main, or run, winding, and the S
refers to the start winding. The coil is
connected between the L and M terminals,
whereas the relay contacts are connected between
the L and S terminals
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Positive Temperature Coefficient Resistors (PTCRs)

• Compressor starting devices that are resistors
  that vary their resistance when their surrounding
  temperature changes
• Have a low resistance over a wide temperature
  range
• Used often in the HVACR industry in place of
  current and potential relays

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Two-Speed Compressor Motors

• Used to control capacity of compressors
• Speed changes made by wiring changes
• The thermostat controls the wiring changes
• Two compressors in one housing
• One motor turns at 1800 rpm the other at 3600
• Two-speed compressors have more than three motor
  terminals

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Special Application Motors

• Facts
• Some single-speed motors have
• More than three motor terminals
• Auxiliary compressor windings to increase the
  motor efficiency
• Have winding thermostats wired through the
  compressor shell
• Three-phase motors have one thermostat for each
  winding, wired in series

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Three-Phase Compressor Motors

• Used in large commercial/industrial applications
• Normally have three motor terminals
• No capacitors are required
• Same resistance across each winding
• High starting torque
• Some larger three-phase compressor motors operate
  as dual voltage device

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Variable Speed Motors

• Motor speed decreases during low load conditions
• Voltage/frequency determine motor speed
• New motors controlled by electronic circuits
• Variable speed DC motors ECM DC motors
• Can ramp up/down to reduce motor wear
• AC current can be converted to DC using rectifiers

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DC Converters (Rectifiers)

• Phase-controlled rectifier
• Converts ac power to dc power
• Uses silicon controlled rectifiers and
  transistors
• Capacitors smooth out the dc voltage

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Rectifiers (contd.)

• Diode bridge rectifier
• Does not regulate the dc voltage
• Diodes are not controllable
• Voltage and frequency are adjusted at the inverter

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Inverters and Variable Frequency Drives (VFDs)

• Vary frequency to obtain desired speed
• Six-step inverter
• Receives voltage from the converter
• Can control the voltage or the current
• Pulse width modulator (PWM)
• Receives fixed dc voltage from converter
• Short pulses at low speed long pulses at high
  speed

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Electronically Commutated Motors (ECM)

• Used on open drive fans less than 1 hp
• Armature commutated with permanent magnets
• Motors are factory calibrated
• Two-piece motor motor section and controls
• Motor can be checked with an ohmmeter
• Controls can be checked with a test module
• Defective controls can be replaced

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Cooling Electric Motors

• All motors must be cooled
• Hermetic compressor motor are cooled by air and
  refrigerant
• Open motors are cooled by air
• Open motors must be located where there is a good
  supply of air
• Some very large motors are cooled by water

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Summary

• Motors facilitate the circulation of air, water,
  refrigerant and other fluids
• Some applications require high starting torque
• Motor components include the housing, rotor,
  stator, end bells, bearings, and motor mount

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Summary (contd.)

• Electricity and magnetism create motor rotation
• Motor speed is determined by the number of poles
• The start winding has higher resistance than the
  run winding
• Important motor amperage are LRA, FLA, and RLA

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Summary (contd.)

• Residences are supplied with single-phase power
• Some motors are designed to operate at more than
  one voltage
• Split phase motors have a medium amount of
  starting torque and good running efficiency

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Summary (contd.)

• The centrifugal switch opens and closes its
  contacts depending on the speed of the motor
• The current relay opens and closes its contacts
  depending on the current flow through the run
  winding

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Summary (contd.)

• The potential relay opens and closes its contacts
  depending on the induced voltage across the start
  winding
• Capacitor start motors use start capacitors to
  increase the starting torque of the motor

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Summary (contd.)

• The start winding and start capacitor are removed
  from the circuit after the motor starts
• Capacitor start, capacitor run motors use both
  start and run capacitors
• Run capacitors help increase the motors running
  efficiency

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Summary (contd.)

• The PSC motor uses only a run capacitor
• The shaded pole motor has very low starting
  torque
• Three-phase motors are used for commercial and
  industrial applications

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Summary (contd.)

• The PTC and NTC are electronic devices that
  change their resistance as the sensed temperature
  changes
• Variable speed motors ramp up and down, often
  using dc converters, inverters and rectifiers
• ECM motors are commutated with permanent magnets

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For more information please contact Mark T. Weber
At North Seattle Community College WWW.NorthSeat
tle.edu Mark.weber_at_seattlecolleges.edu